Update...case...when...then

I'm trying to update the table [faq].
This works ok (no order change).

UPDATE faq SET orderID = CASE id
WHEN 1 then 0
WHEN 2 then 1
WHEN 3 then 2
WHEN 5 then 3
WHEN 4 then 4
WHEN 6 then 5
ELSE orderID END

But when I try to change the order, like this:

UPDATE faq SET orderID = CASE id
WHEN 1 THEN 0
WHEN 2 THEN 1
WHEN 3 THEN 2
WHEN 5 THEN 3
WHEN 4 THEN 4
WHEN 6 THEN 5
ELSE orderID END

I get this error:
Duplicate entry '4' for key 'orderID'

How is that?

Thanks

found a solution.

because MySQL does the update LIVE, it happens to be that duplications may accure.
The solution is ofcourse to make the orderID index not unique.

Wouldn't it just be easier to make the ID unique and auto-increment and then not worry about its value?

Making an ID non-unique is usually a bad idea.